There are many applications where heat must be transferred between objects having different temperatures. The electronics industry, for example, must transfer heat from an electronic module or other high-heat generating device to a heat sink or similar heat-dissipating device.
Several methods of transferring heat between devices have been attempted, but all have significant limitations. One such attempt involves attaching a heat sink directly to the heat-generating device. In such a system, the concern is to maximize the conductivity of the heat sink in proportion to its weight. Generally, if the heat sink material is made lightweight, its heat conductivity is too low. If a better heat conductor such as copper is used, weight becomes a problem. Most importantly, attaching the heat sink directly to the heat source allows one device to transfer loads or stress to the other device. Such stress may be caused by physical movement of one device as a result of shock or vibration; or thermal stress which induces strains. This thermal stress is caused by the difference in the coefficients of thermal expansion of the heat source and the heat sink. Further attempts have been made at reducing the stress transferred between two devices by providing flexible heat conductors. For example, straps or woven strands of a conductive material such as copper may be utilized. Although these flexible heat conductors have conductivity-to-weight ratios much higher than that of the corresponding solid material, they are still not efficient heat conductors and have some rigidity to transfer stress.
Pumping systems have also been used in an attempt to transfer heat between two or more objects. Pumping a fluid medium within such a system, however, requires a source of power to drive the pump. In addition, such systems often exchange a product of the heat transfer into the environment. Operation life is limited by the amount of fuel to be stored. Such pumps also pose problems of torque on space structures.
Another device for heat transfer involves a wicking device, more commonly referred to as a heat pipe. Although these devices are passive, they do require a capillary pumping action. The devices are complex, difficult to fabricate, and sensitive to gravity. In addition, there is an upper limit on the amount of heat transfer that can take place between such a heat pipe and another device. Finally, there is the consideration that such heat pipes can suffer low-grade, long-term chemical degradation, imposing a limit on the useful life of such systems.